As an optical amplifier which directly amplifies an optical signal, there are a semiconductor laser amplifier utilizing a semiconductor laser such as a Fabry-Perot type semiconductor optical amplifier or a traveling wave type optical amplifier (hereinafter referred to as laser diode (LD) amplifier), and an Er doped optical fiber amplifier which comprises a core of optical fiber into which erbium as rare-earth element is doped.
FIG. 3(a) shows a conventional traveling waveguide type optical amplifier. FIG. 3(a) shows a cross-sectional structure and FIGS. 3(b) to 3(g) show the characteristics thereof. In FIG. 3(a), reference numeral 3 designates a traveling waveguide type optical amplifier which receives signal light 11 of a wavelength .lambda. and outputs the amplified output light 12. This traveling waveguide type optical amplifier has a structure which is obtained by coating both facets of the semiconductor laser section 31 with a low reflectance film 32.
The above described semiconductor laser section 31 has a laminated structure which is obtained by producing an n type InP cladding layer 35, an InGaAsP active layer 36, a p type InP cladding layer 37 and a p type InGaAsP contact layer 38 on an n type or p type InP substrate 33 about 80 microns in thickness and producing a p side electrode 39a and an n side electrode 39b at the surface side and the rear surface side of the substrate. The total thickness of the laser section 31 is about approximately 100 microns. Here, the active layer 36 and the cladding layers 35 an 37 constitute a waveguide 34 of the above described semiconductor laser section 31.
The low reflectance film 32 is obtained by producing a silicon oxide film or a silicon nitride film at the facet of the semiconductor laser section 31 to a film thickness of .lambda./4 and lowers the reflectance at the facet of the semiconductor laser section 31 to approximately 1%. This is because the semiconductor laser section 31 itself has a reflectance at the facet of approximately 30%, so that it constitutes a resonant structure and functions as a laser oscillator unless the reflectance at the facet is lowered.
FIG. 3(b) shows an optical output--current characteristics. In figure, the line A shows a characteristic of this traveling waveguide optical amplifier 3, that is, the semiconductor laser section 31 on which a low reflectance film is coated, the dotted line B shows a characteristic of a semiconductor laser section 31 on which low reflectance film is not coated, and reference character Ith designates a threshold of laser oscillation.
An operation will be described.
A signal light 11 is collected by using a lens system (not shown) and put into the built-in waveguide 34 of the semiconductor laser section 31. When two or three times the current of the threshold current value Ith flows before low reflectance film coating is applied to the semiconductor laser section 31, induced emission occurs and the power of the signal light 11 gradually increases along the waveguide 34. Amplication of the signal light 11 is carried out and output light 12 which is amplified is obtained. Here, since the semiconductor laser section 31 is subjected to a low reflectance coating at the both sides facets, it has characteristics as shown by line A, that is, it does not oscillate.
Here, the Fabry-Perot type semiconductor optical amplifier has the same structure as that of the semiconductor laser section 31, and this is biased below the threshold Vth. However, since there are a lot of problems in characteristics such as the small signal gain, this type of amplifier is not frequently used at present.
FIG. 4 shows another example of a traveling waveguide type semiconductor optical amplifier shown in OQE-89 P49, and in this kind of amplifier, the remaining facet reflectance is intended to be reduced by providing a window region at both facets of the active region of a semiconductor laser.
That is, at a central portion of the n type InP substrate 41, an island region 44 comprising a substrate region 41 as an n type cladding layer, an InGaAsP layer 42 as an active layer, and an InP layer 43 as a p type cladding layer is produced. At the both end portions of the substrate 41, window regions 47, 48 of a structure in which a p type InP layer 43, an n type InP layer 45 and a p type InP layer 46 are laminated on the n type InP substrate 41 are produced. Furthermore, a low reflectance film 49 is produced at the both facets of the substrate 41.
In addition, FIGS. 5(a) and 5(b) show another example of a conventional optical amplifier such as an Er doped optical fiber amplifier which is recited in OplusE No. 113 P75. FIG. 5(a) shows a schematic construction of the conventional optical amplifier, FIG. 5(b) shows a cross-sectional structure of an optical fiber thereof, and figures 5(c) to 5(e) show the characteristics thereof. In FIGS. 5, reference numeral 50 designates an Er doped optical fiber amplifier which comprises an Er doped optical fiber 1, a semiconductor laser (hereinafter referred to as `LD`) 2 as an excitation light source of the optical fiber 1, a coupler 4 for coupling the signal light 11 and the excited light, and a filter 5 for separating the amplified light 12 and the excited light 11a.
The above-described optical fiber 1 is constituted of a core 1a which comprises core material of silicon dioxide (SiO.sub.2) to which erbium (Er) is doped, a cladding layer 1b comprising such as glass fiber for covering the core 1a, and a protecting film 1c covering the surface of the cladding layer 1b. The semiconductor laser 2 has the same structure as that of the semiconductor laser section 31 of the above-described traveling wave type optical amplifier. The coupler 4 is provided between the semiconductor laser 2 and the optical fiber 1 and th filter 5 is arranged at the light output side of the optical fiber 1.
In the Er doped optical fiber amplifier of such construction, when signal light 11 enters the optical fiber 1 in a state where the Er atom in the core 1a of the optical fiber 1 is excited to a high energy level by the excited light (wavelength 1.48 microns) of the semiconductor laser 2, an induced emission occurs and the power of the signal light 11 increases gradually along the optical fiber 1 and the amplification of signal light 11 is carried out.
Here, there are a plurality of absorption bands in the Er doped optical fiber 1 as shown in FIG. 5(e) and they can be all utilized as the wavelength of excited light, and the bands of 0.67 microns, 0.98 microns, and 1.48 microns are generally favorable. In addition, as an excitation light source, a dye laser or a gas laser may be utilized instead of a semiconductor laser.
In addition, this optical fiber amplifier has characteristics such as low noise, no dependency of the gain on polarization, and less coupling loss with a transmission line than an LD amplifier using a semiconductor laser.
The characteristics of the semiconductor laser amplifier (LD amp) and an optical fiber amplifier (FA amp) are shown by comparison in the following table.
TABLE ______________________________________ LD amp FA amp ______________________________________ wavelength in use 0.8.about.1.6 microns 1.55 microns 3dB band width 70nm 5nm gain .about.20dB .about.35db saturated output .about.several tens of mW .about.1mW dependency on exist none polarization coupling loss .about.6dB .about.0dB excited current/light current 10mA.about. light 20mW.about. dependency on exist none temperature noise characteristics exist none element length .about.500 microns .about.10 m ______________________________________
These differences in such characteristics are due to the difference between the structure of LD amplifier and that of FA amplifier. The width of wavelength .lambda. of the signal light and the 3 dB band width are determined by the composition of the active layer and the gain and the saturated output can be increased by increasing the element length, the excited current and the excited light.
As for the dependency on polarization, there is a deviation between TM mode and TE mode in the output light as shown in FIG. 3(c) for the LD amplifier, while in the FA amplifier, the different polarization modes coincide with each other as shown in FIG. 5(c).
As for the coupling loss, in the LD amplifier, it is necessary to collect the transmission light by a lens, and the coupling loss becomes approximately 6 dB, while in the FA amplifier, the coupling is between optical fibers, thereby resulting in almost no loss.
In addition, there is a dependency on temperature in the LD amplifier because this LD amplifier comprises semiconductor material (refer to FIG. 3(e)).
This comparison shows that except for the wavelength which can be used as a signal light, and 3dB gain band width and saturated output, the optical fiber amplifier is superior to the LD amplifier. In addition, the modification of the kind and amount of rare-earth dopant element in the optical fiber improved the wavelength of the signal light and 3dB gain band width, and the increased excited light power further enhanced the gain and the saturated output.
As a method for increasing the excited light power, one intending to enhance the outputs by polarization multiplying of two LD amplifiers is shown in Japanese Patent Laid-Open Publication No. 63-115154.
An application example of an optical amplification system utilizing the above-described optical amplifier will be described with reference to FIGS. 6(a) to 6(c).
FIG. 6(a) shows an example of optical pre-amplifier which amplifies the signal light 11 directly before the detector 62. In a case where the signal source is located far away, the signal light 11 is attenuated by transmission path and therefore it is erroneously detected at the detector 62. In such a case, an amplifier 61 is arranged directly before the detector 62, and the attenuated signal light 11 is amplified directly before the detector 62, thereby to prevent the erroneous detection and improve the receiving sensitivity.
FIG. 6(b) shows a construction of an optical amplifier repeater which directly amplifies the signal light which is transmitted and attenuated in the optical fiber and again sends it out to the optical fiber. Here, optical amplifiers 61 are arranged on a transmission path comprising optical fibers (not shown), at predetermined intervals, and the transmitted light which is amplified by the optical amplifier 61 extends the regenerative repeating interval, that is transmission distance.
FIG. 6(c) shows an optical booster amplifier for compensating element insertion loss and optical branching loss in an optical circuit and it is possible to obtain a large sized optical signal processing system in which the size is not restricted by the optical loss. That is, an optical amplifier 61 is arranged at the former or latter stage of an optical branch (optical switch) 63 which branches the signal light 11 and the input signal light 11 is amplified and the branched output light 11b is amplified.
In order to obtain an optical amplifier which can satisfy these uses, the following are required:
(1) small signal gain is larger than 20dB PA0 (2) the gain band width is wide PA0 (3) the signal gain does not depend on the plane of polarization PA0 (4) saturation gain is large PA0 (5) low noise
In addition, it is necessary to have a reliability of about one hundred thousand hours similarly as in the semiconductor laser.
However, in the optical amplifier of FIG. 3(a) which is obtained by low reflectance film coating of both facets of the semiconductor laser, there are following problems.
Firstly, unless the facet reflectance is suppressed to be below 0.01%, a resonant structure is constructed by the residual facet reflectance and the gain band width becomes width W.sub.1, and it becomes narrower than the width W.sub.0 in a case where the resonant structure is not produced. (refer to FIG. 3(d)).
Secondly, the gain largely changes relative to the temperature and the wavelength of the traveling light (refer to FIG. 3(e)).
Thirdly, the saturated output light intensity is low.
As a fourth problem, in order to achieve a non-reflection coating of reflectance of below 0.01%, the tolerance on the refractive index and film thickness of the film used for the non-reflection coating is quite severe, and a non-reflection film cannot be obtained stably and easily. In other words, supposing that the refractive index of the active layer be n.sub.s, the refractive index of the low reflection;,coating film has to satisfy the relation of n=.sqroot.n.sub.s, usually n.sub.s is approximately 3.4, so that the value X of SiO.sub.x and SiN.sub.x which constitute the low reflection coating film have to be adjusted so that the n is approximately in a range of 1.85.+-.0.01. Furthermore, the film thickness d of the low reflection coating film has to be approximately .lambda./4n.+-.10 angstroms (refer to FIG. 3(f)).
Furthermore, in the traveling wave type semiconductor optical amplifier having a window region shown in FIG. 4, by using a window structure, it is possible to suppress the facet reflectance to be below 0.015, but it was really difficult to produce a semiconductor laser having a window structure with high reproducibility due to the complexity in the structure.
In addition, in an optical fiber amplifier using an optical fiber with a rare-earth element dopant such as Er, a dye laser, a gas laser, and a semiconductor laser are used as an excitation light source, but in all of those except the semiconductor laser the apparatus becomes unfavorably large. Furthermore, as shown in FIG. 5(d), although the gain band width is W.sub.2 at an excitation light power of 20 mW, the gain band width becomes W.sub.3 (&gt;W.sub.2) at an excitation light power 50 mW, and in order to increase the gain and obtain a wide band width, the excitation power has to be increased. As for the above-described semiconductor laser, the semiconductor laser itself is at stage of research for 0.67 microns and 0.98 microns band, there is no semiconductor laser which can be used as the excitation light source. In 1.48 microns band, there is a semiconductor laser having a light output of about 100 mW. However, when it is used at a power output above 100 mW, sufficient reliability is not obtained relative to a light output level (.about.10 mW) which has been used in the optical communication until now.
In a method of obtaining high output by polarization multiplying of two LD amplifiers as recited in Japanese Patent Laid-Open Publication No. 63-115154, in order to obtain an output of 100 mW,, one LD has to be at 50 mW and this also results in a problem in reliability.